[Uptime] Ping history table enhancements

[formgeist]

Summary

The ping history table which is shown on the detail page of a specific browser monitor has a few visual bugs that can be fixed by enhancing the visual styles a bit.

1. Set a fixed height on the image container for the preview thumbnail in the row item

We should fix the height of the image container to ensure the rows don't jump on load. We can fix this to 64px.

2. Reduce the size of the loading spinner inside the image container when previews are loading

We can reduce the size to size="m".

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)